Methods, systems, and apparatus for mass flow verification based on choked flow

ABSTRACT

Mass flow verification systems and apparatus may verify mass flow rates of mass flow controllers (MFCs) based on choked flow principles. These systems and apparatus may include a plurality of differently-sized flow restrictors coupled in parallel. A wide range of flow rates may be verified via selection of a flow path through one of the flow restrictors based on an MFC&#39;s set point. Mass flow rates may be determined via pressure and temperature measurements upstream of the flow restrictors under choked flow conditions. Methods of verifying a mass flow rate based on choked flow principles are also provided, as are other aspects.

FIELD

This disclosure relates to electronic device manufacturing and, moreparticularly, to verifying mass flow rates of mass flow controllersbased on choked flow principles.

BACKGROUND

Electronic device manufacturing systems may include one or more massflow controllers (MFCs). MFCs control the mass flow rates of processchemistries used in the manufacture of electronic devices. Processchemistries may include various gases (e.g., cleaning, deposition, andetchant gases) that are delivered to one or more process chambers inwhich electronic circuits may be fabricated on semiconductor wafers,glass plates, or the like. Precise mass flow control of processchemistries may be used in one or more steps of an electronic device'sfabrication process. Precise mass flow control provided by MFCs maycontribute to high yield production of electronic devices havingmicroscopically small dimensions.

To ensure that process chemistries are delivered accurately,verification and calibration of MFC's may be performed periodically.However, verifying and calibrating MFCs may involve additional bulky andexpensive equipment that may be time-consuming and inefficient to use,may be limited to low mass flow rate ranges (e.g., up to only 3000 sccm(standard cubic centimeter per minute) nitrogen equivalent), may resultin notable process downtime, and/or may not be sufficiently accurate toensure precise mass flow control of process chemistries.

SUMMARY

According to a first aspect, a mass flow verification system isprovided. The mass flow verification system comprises an inlet; a firstpressure sensor and a temperature sensor each coupled downstream of theinlet; a plurality of isolation valves coupled downstream of the inlet;a plurality of differently-sized flow restrictors coupled in paralleland downstream of the inlet, each one of the plurality ofdifferently-sized flow restrictors coupled in series with the inlet anda respective one of the plurality of isolation valves; an outlet coupleddownstream of and in series with each one of the plurality ofdifferently-sized flow restrictors; and a controller coupled to thefirst pressure sensor, the temperature sensor, and the plurality ofisolation valves, wherein the controller is configured to determine amass flow rate in response to a temperature measured by the temperaturesensor under a choked flow condition and a first pressure measured bythe first pressure sensor under the choked flow condition.

According to a second aspect, an electronic device manufacturing systemis provided. The electronic device manufacturing system comprises a massflow controller; a mass flow verification system having an inlet and anoutlet, the inlet coupled to the mass flow controller; a controller; anda process chamber coupled to a flow path coupled to the mass flowcontroller and configured to receive one or more process chemistries viathe mass flow controller. The mass flow verification system comprises aplurality of isolation valves coupled downstream of the inlet, and aplurality of differently-sized flow restrictors coupled in parallel anddownstream of the inlet, each one of the plurality of differently-sizedflow restrictors coupled in series with the inlet, a respective one ofthe plurality of isolation valves, and the outlet. The controller iscoupled to the plurality of isolation valves and is configured toreceive a pressure measurement and a temperature measurement upstream ofthe plurality of differently-sized flow restrictors under a choked flowcondition through only one of the plurality of isolation valves. Thecontroller is also configured to determine a mass flow rate in responseto receiving the pressure measurement and the temperature measurement.

According to a third aspect, a method of verifying a mass flow rate isprovided. The method comprises causing a gas to flow through only one ofa plurality of differently-sized flow restrictors during a choked flowcondition; measuring a pressure upstream of the one of the plurality ofdifferently-sized flow restrictors to obtain a measured pressure valueduring the choked flow condition; measuring a temperature upstream ofthe one of the plurality of differently-sized flow restrictors to obtaina measured temperature value during the choked flow condition; anddetermining a mass flow rate by applying a predetermined flow restrictorcoefficient, a predetermined gas correction factor, and a predeterminedtemperature value to the measured pressure value and the measuredtemperature value.

Still other aspects, features, and advantages in accordance with theseand other embodiments of the disclosure may be readily apparent from thefollowing detailed description, the appended claims, and theaccompanying drawings. Accordingly, the drawings and descriptions hereinare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

The drawings, described below, are for illustrative purposes only andare not necessarily drawn to scale. The drawings are not intended tolimit the scope of the disclosure in any way.

FIG. 1 illustrates a first mass flow verification system according toembodiments of the disclosure.

FIG. 2 illustrates a second mass flow verification system according toembodiments of the disclosure.

FIG. 3 illustrates a third mass flow verification system according toembodiments of the disclosure.

FIG. 3A illustrates a graph of several pressures during mass flowverification within the third mass flow verification system according toembodiments of the disclosure.

FIG. 4 illustrates a fourth mass flow verification system according toembodiments of the disclosure.

FIG. 5 illustrates an electronic device manufacturing system accordingto embodiments of the disclosure.

FIG. 6 illustrates a method of mass flow verification according toembodiments of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to example embodiments of thedisclosure, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Electronic devices having microscopically small dimensions may beproduced with process chemistries having mass flow rate accuracies ashigh as +/− 1%. Many mass flow controllers (MFCs) may be specified assuch and may meet those specifications when new, while a smallpercentage of MFCs may be specified as such, but may not actually meetthem when new or otherwise. Furthermore, even initially accurate MFCsmay over time experience an accuracy drift in their mass flow rates thatmay render them outside of their specified accuracies. Accordingly,verification and calibration of MFCs, such as those used insemiconductor fabrication equipment, may be performed periodically toensure that process chemistries are delivered accurately.

Mass flow verification methods, systems, and apparatus in accordancewith one or more embodiments of the disclosure are based on choked flowprinciples for determining a gas mass flow rate, which may be in unitsof “sccm” (standard cubic centimeters per minute) or “slm” (standardliters per minute). Mass flow verification methods, systems, andapparatus based on choked flow principles in accordance with one or moreembodiments of the disclosure may reduce the number of variables neededto calculate mass flow rate, may result in a smaller verificationequipment footprint, and may be more time efficient and at least asaccurate as, if not more accurate than, mass flow verification methods,systems, and apparatus based on known ROR (pressure rate of rise)principles.

ROR principles are based on ideal gas law to correlate a mass flow ratewith a measured pressure rate of rise in a known enclosed volume. Thehigher the mass flow rate, the larger (should be) the enclosed volume.ROR principles may involve a lengthy process (e.g., 10 or more hours insome cases) of filling an enclosed volume with a gas and measuring apressure rate of rise within the enclosed volume. The enclosed volumemay be a process chamber of a manufacturing system or an externalvolume. Uncertainties in the exact volume of a process chamber orexternal volume may adversely affect the accuracy of the results. Aprocess using ROR principles may involve measurements of pressure,temperature, volume, and time.

In contrast, non-ROR choked flow measurement may be almostinstantaneous, and calculating a mass flow rate based on choked flowprinciples may involve just two measurements—pressure and temperature.

According to choked flow principles, the velocity of a gas flowingthrough a restrictive pathway (e.g., the narrowest part of the pathway)initially increases as the pressure difference across the restrictionincreases. Choked flow occurs when the pressure difference becomes largeenough to increase gas flow velocity to the speed of sound (i.e., sonicvelocity) at the restriction, or choke point. In other words, chokedflow occurs at a particular minimum ratio of pressure upstream of thechoke point to pressure downstream of the choke point. During chokedflow, the velocity of the gas does not increase above the speed of soundat the choke point no matter how large the pressure difference becomes.A choke point may be provided by a device known as a flow restrictor.Flow restrictors are available in many different sizes (i.e., thediameter or cross-sectional area of the orifice or flow path through therestrictor).

Mass flow rate (MFR) through a flow restrictor during choked flow varieslinearly with pressure upstream of the flow restrictor. Mass flowverification in accordance with one or more embodiments of thedisclosure may employ the following two equations:FlowSTD MFR=PCHARACTERIZATION*CFLOWRESTR  (1)Verified MFR=PMFV*CFLOWRESTR*(TCHARACTERIZATION/TMFV)*CGASCORRECTION  (2)

where:

Equation (1) is used during a characterization (described below) of eachof the differently-sized flow restrictors used in mass flow verificationsystems in accordance with one or more embodiments of the disclosure;

Equation (2) is used during mass flow verification of an MFC todetermine a mass flow rate (i.e., Verified MFR);

temperature and pressure values are in absolute units (i.e., degreesKelvin for temperature values and Torr for pressure values);

nitrogen may be a reference gas used to characterize thedifferently-sized flow restrictors;

PCHARACTERIZATION is a measured upstream pressure of nitrogen flowing ata known mass flow rate (i.e., FlowSTD MFR) through a given flowrestrictor under a choked flow condition during characterization of thatgiven flow restrictor;

CFLOWRESTR (flow restrictor coefficient) is a ratio of a known mass flowrate (FlowSTD MFR) over a measured pressure (PCHARACTERIZATION) for agiven flow restrictor as calculated in Equation 1; values of CFLOWRESTRfor each characterized flow restrictor may be stored in a memory of acontroller configured to control a mass flow verification system inaccordance with one or more embodiments of the disclosure; CFLOWRESTR isused in Equation 2 to solve for Verified MFR;

TCHARACTERIZATION is a measured temperature of nitrogen duringcharacterization of a given flow restrictor; values of TCHARACTERIZATIONfor each characterized flow restrictor may be stored in a memory of acontroller configured to control a mass flow verification system inaccordance with one or more embodiments of the disclosure;

PMFV is a measured upstream pressure of a target gas (i.e., the processchemistry whose mass flow rate is controlled by an MFC being verified)under a choked flow condition through a given flow restrictor duringmass flow verification, wherein the given flow restrictor should havebeen previously characterized;

TMFV is a measured upstream temperature of the target gas under thechoked flow condition through the given flow restrictor during mass flowverification; and

CGAS CORRECTION (gas correction factor) is the square root of a ratio ofthe molecular weight of nitrogen over the molecular weight of the targetgas (i.e., the process chemistry whose mass flow rate is controlled byan MFC being verified). This factor corrects for differences betweennitrogen and non-nitrogen gases that may be flowed during mass flowverification, and values of CGAS CORRECTION for various gases that maybe used during mass flow verification may be stored in a memory of acontroller configured to control a mass flow verification system inaccordance with one or more embodiments of the disclosure.

Mass flow verification methods, systems, and apparatus in accordancewith one or more embodiments of the disclosure may use a plurality ofdifferently-sized flow restrictors coupled in parallel to induce chokedflow for a mass flow rate range of an MFC used in a gas deliveryapparatus of an electronic device manufacturing system. Choked flowconditions may be created by maintaining a minimum upstream/downstreampressure ratio across the plurality of differently-sized flowrestrictors. The minimum upstream/downstream pressure ratio that induceschoked flow may be known or can be determined for each of the gases usedin electronic device manufacturing. The number of differently-sized flowrestrictors is determined by the range of mass flow rates to beverified; the wider the flow rate range, the more differently-sized flowrestrictors may be included. Embodiments of the methods, systems, andapparatus described herein can be expanded to accommodate various massflow rate ranges used in electronic device manufacturing.

Each differently-sized flow restrictor may be characterized via a testsetup to measure a resulting upstream pressure during a choked flowcondition for one or more mass flow rates to be verified. This mayensure that an upstream pressure of a particular flow restrictor thatmay be used in a mass flow verification system in accordance withembodiments of the disclosure does not starve an MFC to be verified.That is, in order to function properly, an MFC should have a certainpressure differential thereacross. If an upstream pressure resultingfrom a particular mass flow rate through a particular flow restrictorcauses an insufficient pressure differential across an MFC, then thatflow restrictor cannot be used to verify that MFC at that mass flowrate. Thus, embodiments of the disclosure employ a plurality ofdifferently-sized flow restrictors coupled in parallel such that anappropriate flow restrictor can be selected to provide a choked flowpath without starving the MFC to be verified.

A test setup for characterizing flow restrictors may include a precisionMFC coupled to an inlet of a flow path having an upstream pressuresensor (e.g., a manometer), an upstream temperature sensor, a flowrestrictor to be characterized, and a downstream pressure sensor (e.g.,a manometer) coupled in series. The outlet of the flow path may becoupled to a vacuum pump to create a base vacuum pressure at the outlet,which may be, e.g., 5 Torr (other base vacuum pressures may be used).Once the base vacuum pressure is established, the precision MFC can beset to a particular mass flow rate (referred to herein as a set point)and nitrogen can be flowed through the flow path. Measurement ofupstream and downstream pressures can then be made to ensure that theminimum upstream/downstream pressure ratio exists to induce choked flowthrough the flow restrictor and that the upstream pressure does notstarve the MFC.

The following are example results of characterized 100-micron,400-micron, and 800-micron flow restrictors each having nitrogen flowedthere through at a base vacuum pressure of 5 Torr for the mass flow rateranges shown:

100 micron flow restrictor

-   -   A) mass flow rate=5 sccm        -   upstream pressure=71.4 Torr        -   downstream pressure=5.0 Torr    -   B) mass flow rate=45 sccm        -   upstream pressure=621.5 Torr        -   downstream pressure=5.0 Torr

400 micron flow restrictor

-   -   A) mass flow rate=30 sccm        -   upstream pressure=26.2 Torr        -   downstream pressure=5.0 Torr    -   B) mass flow rate=700 sccm        -   upstream pressure=600.6 Torr        -   downstream pressure=5.7 Torr

800 micron flow restrictor

-   -   A) mass flow rate=400 sccm        -   upstream pressure=86.1 Torr        -   downstream pressure=5.2 Torr    -   B) mass flow rate=3000 sccm        -   upstream pressure=648.1 Torr        -   downstream pressure=12.7 Torr

The results for each characterized flow restrictor may be stored in amemory of a controller controlling the mass flow verification system.These results may allow the controller to select an appropriately-sizedflow restrictor to be used to verify a mass flow rate of an MFC during achoked flow condition without starving the MFC.

Further details of example embodiments illustrating and describing thevarious aspects above, as well as other aspects including methods ofverifying a mass flow rate, will be explained in greater detail below inconnection with FIGS. 1-6.

Mass flow verification methods, systems, and apparatus in accordancewith one or more embodiments of the disclosure may includereduced-pressure (i.e., vacuum-based) applications employing, e.g., massflow verification systems 100 and 200, and atmospheric (i.e., ambientpressure based) applications employing, e.g., mass flow verificationsystems 300 and 400, as now described.

FIG. 1 illustrates a mass flow verification system 100 in accordancewith one or more embodiments. Mass flow verification system 100 may beused in low-flow reduced-pressure applications. In some embodiments, lowflow applications may include mass flow rates up to about 2500 sccm, forexample.

A mass flow controller (MFC) 99 may be coupled to mass flow verificationsystem 100 at inlet 102 of mass flow verification system 100. In someembodiments, MFC 99 may represent a plurality of MFCs coupled to inlet102 via a common manifold or header having a common outlet, wherein MFC99 as described below may represent the one MFC of the plurality of MFCsto be verified (i.e., the only MFC of the plurality of MFCs flowing gasduring verification). MFC 99 may be a part of, or coupled to, a gasdelivery apparatus of an electronic device manufacturing system. MFC 99may be configured to flow a gas at one or more specified mass flow rates(i.e., one or more set points) to one or more process chambers of theelectronic device manufacturing system. Mass flow verification system100 is configured to verify one or more of the specified mass flow ratesof MFC 99 based on choked flow principles.

Mass flow verification system 100 may include a plurality of isolationvalves 103-110, a temperature sensor 111, a plurality of pressuresensors 112-115, a plurality of differently-sized flow restrictors116-120, a gas temperature acclamation accelerator 121, and an outlet122. Outlet 122 may be coupled to a foreline (i.e., a vacuum line to asystem vacuum pump) of the electronic device manufacturing system toestablish a base vacuum pressure at outlet 122.

The plurality of isolation valves 103-110 may be coupled downstream ofinlet 102. Isolation valves 103 and 105 may be part of a bypass flowpath 123 coupled between inlet 102 and outlet 122 that bypasses theplurality of differently-sized flow restrictors 116-120. Isolationvalves 103 and 105 and bypass flow path 123 may enable pumping andpurging of mass flow verification system 100 after measurement ofhazardous gases that may be provided by gas delivery apparatus via MFC99. Isolation valve 104 may be a main verification system valve.Isolation valves 103-110 may each be any suitableelectronically-controllable isolation valve capable of stopping gas flowthere through across a range of pressures provided by the gas deliveryapparatus and the electronic device manufacturing system to which massflow verification system 100 is connected and by the choked flowconditions created within mass flow verification system 100.

The plurality of differently-sized flow restrictors 116-120 are coupledin parallel and downstream of inlet 102. Each of the differently-sizedflow restrictors 116-120 may be configured for a different maximum flowrate there through (which may be referred to as “conductance”) than theother differently-sized flow restrictors 116-120. For example, in someembodiments, flow restrictor 116 may have the highest flow rate therethrough, while flow restrictor 117 may have a high flow rate therethrough, but less than flow restrictor 116. Flow restrictor 118 may havea medium flow rate there through (i.e., less than flow restrictors 116and 117), while flow restrictor 119 may have a low flow rate therethrough (i.e., less than each of flow restrictors 116-118). And flowrestrictor 120 may have the lowest flow rate there through (i.e., lessthan each of flow restrictors 116-119). In some embodiments, the highestflow rate may be about 5000 sccm and the lowest flow rate may be about 5sccm, for example. In some embodiments, the differently-sized flowrestrictors 116-120 may be precision flow restrictors. In otherembodiments, standard flow restrictors may be used.

As shown in FIG. 1, each one of the differently-sized flow restrictors116-120 is coupled in series with inlet 102 and a respective one ofisolation valves 106-110. That is, flow restrictor 116 is coupled inseries with isolation valve 106, flow restrictor 117 is coupled inseries with isolation valve 107, flow restrictor 118 is coupled inseries with isolation valve 108, flow restrictor 119 is coupled inseries with isolation valve 109, and flow restrictor 120 is coupled inseries with isolation valve 110. In some embodiments as shown, thedifferently-sized flow restrictors 116-120 are coupled upstream of theirrespective isolation valves 106-110.

In other embodiments, the number of differently-sized flow restrictorsand their respective series-coupled isolation valve may be more or lessthan that shown, depending on the range of mass flow rates to beverified by mass flow verification system 100. The greater the range ofmass flow rates to be verified, the larger the number ofseries-connected differently-sized flow restrictor/isolation valvepairs.

Temperature sensor 111 and pressure sensors 112 and 113 may each becoupled downstream of inlet 102 and upstream of differently-sized flowrestrictors 116-120. Pressure sensors 114 and 115 may be coupleddownstream of flow restrictor 116 (i.e., the flow restrictor having thehighest flow rate). Temperature sensor 111 may be a thermocouple, andeach of pressure sensors 112-115 may be a manometer. In someembodiments, temperature sensor 111 may include more than onethermocouple, and one or more of pressure sensors 112-155 may includemore than one manometer. In some embodiments, pressure sensors 112 and115 may each be a 100 Torr manometer, pressure sensor 113 may be a 1000Torr manometer, and pressure sensor 114 may a 10 Torr manometer. Otherembodiments may have pressure sensors of other Torr values. Furthermore,some embodiments may have only one of pressure sensors 112 and 113 andonly one of pressure sensors 114 and 115, depending on the range of massflow rates to be verified by mass flow verification system 100.

The gas temperature acclamation accelerator 121 may be coupled upstreamof temperature sensor 111. The gas temperature acclamation accelerator121 may be used to ensure uniform gas temperature distribution upstreamof the flow restrictors 116-120, which may improve the accuracy of massflow verification system 100. The gas temperature acclamationaccelerator 121 may be an inactive structure that includes a porous meshmaterial having an optimum amount of surface area that may result innegligible, if any, pressure drop there through.

Mass flow verification system 100 may further include a controller 124.Controller 124 may control the operation of and be electronically (orotherwise) coupled to isolation valves 103-110, temperature sensor 111,and pressure sensors 112-115. Controller 124 may be, e.g., a generalpurpose computer and/or may include a microprocessor or other suitablecomputer processor or CPU (central processing unit) capable of executingcomputer readable instructions/software routines. Controller 124 mayinclude a memory for storing data and computer readableinstructions/software routines executable thereon. Flow restrictorcharacterization data may be stored in the memory of controller 124.

Controller 124 may be configured via user input commands and the storedcomputer readable instructions/software routines to set a set point forMFC 99, select a flow path through one of the differently-sized flowrestrictors 116-120, control the opening and closing of each of theisolation valves 103-110, record and process temperature and pressuremeasurements via temperature sensor 111 and pressure sensors 112-115,and determine mass flow rates based on the recorded temperature andpressure measurements and Equation 2, as described herein. Controller124 may also be configured to control other aspects of mass flowverification system 100, including, e.g., input/output peripherals,power supplies, clock circuits, and/or the like.

In some embodiments, controller 124 may not be included in mass flowverification system 100. Instead, controller 124 may be, e.g., a systemcontroller of an electronic device manufacturing system to which massflow verification system 100 is connected. Data and computer readableinstructions/software routines configured to operate mass flowverification system 100 for verifying mass flow rates as describedherein may be stored on a non-transitory computer-readable medium, suchas, e.g., a removable storage disk or device. The data and computerreadable instructions/software routines may be transferred from thenon-transitory computer-readable medium to the system controller toperform mass flow verification.

Mass flow verification system 100 may be operated by setting MFC 99 viacontroller 124 to a desired mass flow rate (i.e., a desired set point)to be verified and then flowing a gas through the highest flow pathway(i.e., through flow restrictor 116) via controller 124, which mayconnect the highest flow pathway to MFC 99 by closing isolation valves105 and 107-110 and opening isolation valves 103 and 104 (for flowthrough main flow path 101) and isolation valve 106 (for flow throughflow restrictor 116). Pressure measurements may be recorded bycontroller 124 via one of downstream pressure sensors 114 or 115 (thismay depend on the base vacuum pressure set at outlet 122 via a systemvacuum pump connected thereto and on the pressure ratings of pressuresensors 114 and 115—note that in some embodiments, as described above,10 Torr may be the maximum pressure measurable by pressure sensor 114,while 1000 Torr may be the maximum pressure measurable by pressuresensor 113). Similarly, pressure measurements may be recorded bycontroller 124 via pressure sensor 112 or 113. Controller 124 may thendetermine whether the minimum upstream/downstream pressure ratio forinducing choked flow is present. In some embodiments, this may occurwhen upstream pressure is greater than twice the downstream pressure.

Because isolation valves 106-110 are downstream of the differently-sizedflow restrictors 116-120, the highest flow pathway should be initiallyselected first to measure downstream pressure to confirm choked flow.Furthermore, pressure sensors 114 and 115 may only be coupled to thehighest flow pathway, because coupling similar pairs of pressure sensorsdownstream of each of the differently-sized flow restrictors 116-120 maybe cost prohibitive.

In response to determining that a minimum upstream/downstream pressureratio for choked flow is present (i.e., establishing that a choked flowcondition exists), controller 124 may select an appropriate one of thedifferently-sized flow restrictors 116-120 based on storedcharacterization data (by opening and closing corresponding isolationvalves 103-110) to verify a set point of MFC 99. The selected flowrestrictor is one that maintains choked flow there through at the setpoint of MFC 99 to be verified without the resulting upstream pressureof that flow restrictor exceeding a threshold value and causing MFC 99to starve (as determined during characterization). Upstream temperatureand pressure measurements may then be made via temperature sensor 111and one of pressure sensors 112 or 113. Controller 124 may determine amass flow rate using Equation 2. This process may be repeated forverifying other mass flow rates of MFC 99. If a determined mass flowrate is found to be outside of MFC 99's specified accuracy, MFC 99 maybe adjusted (if possible) or replaced.

FIG. 2 illustrates another mass flow verification system 200 inaccordance with one or more embodiments. Mass flow verification system200 may be used in high-flow reduced-pressure applications (i.e.,vacuum-based applications). In some embodiments, high flow applicationsmay include mass flow rates up to about 50 slm, for example, while inother embodiments, no upper mass flow rate limitation may apply.Alternatively, mass flow verification system 200 may also be used inlow-flow reduced-pressure applications.

A mass flow controller (MFC) 99B may be coupled to mass flowverification system 200 at inlet 202 of mass flow verification system200. In some embodiments, MFC 99B may represent a plurality of MFCscoupled to inlet 202 via a common manifold or header having a commonoutlet, wherein MFC 99B as described below may represent the one MFC ofthe plurality of MFCs to be verified (i.e., the only MFC of theplurality of MFCs flowing gas during verification). MFC 99B may be apart of, or coupled to, a gas delivery apparatus of an electronic devicemanufacturing system. MFC 99B may be configured to flow a gas at one ormore specified mass flow rates (i.e., one or more set points) to one ormore process chambers of the electronic device manufacturing system.Mass flow verification system 200 is configured to verify one or more ofthe specified mass flow rates of MFC 99B based on choked flowprinciples.

Mass flow verification system 200 may include a plurality of isolationvalves 203-210 and 226-230, a temperature sensor 211, pressure sensors212 and 214, a plurality of differently-sized flow restrictors 216-220,a gas temperature acclamation accelerator 221, and an outlet 222. Outlet222 may be coupled to a foreline (i.e., a vacuum line to a system vacuumpump) of the electronic device manufacturing system to establish a basevacuum pressure at outlet 222.

The plurality of isolation valves 203-210 and 226-230 may be coupleddownstream of inlet 102. Isolation valves 203 and 205 may be part of abypass flow path 223 coupled between inlet 202 and outlet 222 thatbypasses the plurality of differently-sized flow restrictors 216-220.Isolation valves 203 and 205 and bypass flow path 223 may enable pumpingand purging of mass flow verification system 200 after measurement ofhazardous gases that may be provided by gas delivery apparatus via MFC99B. Isolation valve 204 may be a main verification system valve.Isolation valves 203-210 and 226-230 may each be any suitableelectronically-controllable isolation valve capable of stopping gas flowthere through across a range of pressures provided by the gas deliveryapparatus and the electronic device manufacturing system to which massflow verification system 200 is connected and by the choked flowconditions within mass flow verification system 200.

The plurality of differently-sized flow restrictors 216-220 are coupledin parallel and downstream of inlet 202. Each of the differently-sizedflow restrictors 216-220 is configured to allow a different maximum flowrate there through than the other differently-sized flow restrictors216-220. In some embodiments, for example, flow restrictor 216 may havethe highest flow rate there through, while flow restrictor 217 may havea high flow rate there through, but less than flow restrictor 216. Flowrestrictor 218 may have a medium flow rate there through (i.e., lessthan flow restrictors 216 and 217), while flow restrictor 219 may have alow flow rate there through (i.e., less than each of flow restrictors216-218). And flow restrictor 220 may have the lowest flow rate therethrough (i.e., less than each of flow restrictors 216-219). In someembodiments, the differently-sized flow restrictors 216-220 may beprecision flow restrictors. In other embodiments, standard flowrestrictors may be used.

As shown in FIG. 2, each one of the differently-sized flow restrictors216-220 is coupled in series with inlet 202 and a respective one ofisolation valves 206-210. That is, flow restrictor 216 is coupled inseries with isolation valve 206, flow restrictor 217 is coupled inseries with isolation valve 207, flow restrictor 218 is coupled inseries with isolation valve 208, flow restrictor 219 is coupled inseries with isolation valve 209, and flow restrictor 220 is coupled inseries with isolation valve 210. In some embodiments as shown, thedifferently-sized flow restrictors 216-220 are coupled downstream oftheir respective isolation valves 206-210.

In other embodiments, the number of differently-sized flow restrictorsand their respective series-coupled isolation valve may be more or lessthan that shown depending on the range of mass flow rates to be verifiedby mass flow verification system 200. The greater the range of mass flowrates to be verified, the larger the number of series-connecteddifferently-sized flow restrictor/isolation valve pairs.

Temperature sensor 211 and pressure sensor 212 may each be coupleddownstream of inlet 202 and upstream of the differently-sized flowrestrictors 216-220. Pressure sensor 214 may be coupled downstream ofthe differently-sized flow restrictors 216-220. As shown in FIG. 2, eachone of the sub-plurality of isolation valves 226-230 has a respectivefirst port 236-239 (except isolation valve 230 which shares first port239 with isolation valve 229) coupled to temperature sensor 211 and topressure sensor 212. Each one of the sub-plurality of isolation valves226-230 also has a second port coupled between a respective one of thedifferently-sized flow restrictors 216-220 and a respective one ofisolation valves 206-210. That is, isolation valve 226 has a second port246 coupled between flow restrictor 216 and isolation valve 206,isolation valve 227 has a second port 247 coupled between flowrestrictor 217 and isolation valve 207, isolation valve 228 has a secondport 248 coupled between flow restrictor 218 and isolation valve 208,isolation valve 229 has a second port 249 coupled between flowrestrictor 219 and isolation valve 209, and isolation valve 230 has asecond port 250 coupled between flow restrictor 220 and isolation valve210.

In some embodiments, the sub-plurality of isolation valves 226-230 mayeach be a suitable mini-valve configured to allow temperature sensor 211and pressure sensor 212 to accurately measure temperature and pressure,respectively, upstream of each one of the differently-sized flowrestrictors 216-220 when its corresponding isolation valve 226-230 isopen.

Temperature sensor 211 may be a thermocouple, and pressure sensors 212and 214 may each be a manometer. In some embodiments, temperature sensor211 may include more than one thermocouple, and pressure sensor 212and/or 214 may include more than one manometer. In some embodiments,pressure sensor 212 may be a 1000 Torr manometer, and pressure sensor214 may a 10 Torr manometer. Other embodiments may have pressure sensorsof other Torr values and/or may have more than two pressure sensors,depending on the range of mass flow rates to be verified by mass flowverification system 200.

The gas temperature acclamation accelerator 221 may be coupled upstreamof temperature sensor 211. The gas temperature acclamation accelerator221 may be used to ensure uniform gas temperature distribution upstreamof the flow restrictors 216-220, which may improve the accuracy of massflow verification system 200. The gas temperature acclamationaccelerator 221 may be an inactive structure that includes a porous meshmaterial having an optimum amount of surface area that may result innegligible, if any, pressure drop there through.

Mass flow verification system 200 may further include a controller 224.Controller 224 may control the operation of and be electronically (orotherwise) coupled to isolation valves 203-210 and 226-230, temperaturesensor 211, and pressure sensors 212 and 214. Controller 224 may be,e.g., a general purpose computer and/or may include a microprocessor orother suitable computer processor or CPU (central processing unit)capable of executing computer readable instructions/software routines.Controller 224 may include a memory for storing data and computerreadable instructions/software routines executable thereon. Flowrestrictor characterization data may be stored in the memory ofcontroller 224.

Controller 224 may be configured via user input commands and the storedcomputer readable instructions/software routines to set a set point forMFC 99B, select a flow path through one of the differently-sized flowrestrictors 216-220, control the opening and closing of each of theisolation valves 203-210 and 226-230, record and process temperature andpressure measurements via temperature sensor 211 and pressure sensors212 and 214, and determine mass flow rates based on the recordedtemperature and pressure measurements and Equation 2, as describedherein. Controller 224 may also be configured to control other aspectsof mass flow verification system 200, including, e.g., input/outputperipherals, power supplies, clock circuits, and/or the like.

In some embodiments, controller 224 may not be included in mass flowverification system 200. Instead, controller 224 may be, e.g., a systemcontroller of an electronic device manufacturing system to which massflow verification system 200 is connected. Data and computer readableinstructions/software routines configured to operate mass flowverification system 200 for verifying mass flow rates as describedherein may be stored on a non-transitory computer-readable medium, suchas, e.g., a removable storage disk or device. The data and computerreadable instructions/software routines may be transferred from thenon-transitory computer-readable medium to the system controller toperform mass flow verification.

Mass flow verification system 200 may be operated by setting MFC 99B viacontroller 224 to a desired mass flow rate (i.e., a desired set point)to be verified, selecting via controller 224 an appropriate flow paththrough one of differently-sized flow restrictors 216-220 based onstored characterization data (wherein controller 224 opens and closesthe appropriate isolation valves 203-210 and 226-230), taking downstreampressure measurements via pressure sensor 214 (to confirm choked flow)and upstream temperature and pressure measurements via temperaturesensor 211 and pressure sensor 212, and determining via controller 224 amass flow rate using Equation 2. This process may be repeated forverifying other mass flow rates of MFC 99B. If a determined mass flowrate is found to be outside of MFC 99B's specified accuracy, MFC 99B maybe adjusted (if possible) or replaced. Note that unlike mass flowverification system 100, flowing gas through the highest flow path firstis not done in mass flow verification system 200 because pressure sensor214 is coupled such that downstream pressure may be directly measured ateach of differently-sized flow restrictors 216-220.

FIG. 3 illustrates another mass flow verification system 300 inaccordance with one or more embodiments. Mass flow verification system300 may be used in atmospheric applications (i.e., non-vacuumapplications) and, alternatively, may be also in reduced-pressureapplications, as described further below.

A mass flow controller (MFC) 99C may be coupled to mass flowverification system 300 at inlet 302 of mass flow verification system300. In some embodiments, MFC 99C may represent a plurality of MFCscoupled to inlet 302 via a common manifold or header having a commonoutlet, wherein MFC 99C as described below may represent the one MFC ofthe plurality of MFCs to be verified (i.e., the only MFC of theplurality of MFCs flowing gas during verification). MFC 99C may be apart of, or coupled to, a gas delivery apparatus of an electronic devicemanufacturing system. MFC 99C may be configured to flow a gas at one ormore specified mass flow rates (i.e., one or more set points) to one ormore process chambers of the electronic device manufacturing system.Mass flow verification system 300 is configured to verify one or more ofthe specified mass flow rates of MFC 99C based on choked flowprinciples.

Mass flow verification system 300 may include a plurality of isolationvalves 303-310, 331, and 333; a temperature sensor 311; pressure sensors312 and 314, a plurality of differently-sized flow restrictors 316-320,a gas temperature acclamation accelerator 321, an outlet 322, a dead-endtank 325, a vacuum pump 332, and an input port 334.

Input port 334 may be coupled to a source of CDA (clean dry air) ornitrogen. Vacuum pump 332 may be, e.g., a venturi vacuum generator, andmay have an exhaust port 335 that may be coupled to an abatement systemof an electronic device manufacturing system or other suitable apparatusfor receiving discharged gases. Dead-end tank 325 may have a volume of,e.g., about 25 liters. Dead-end tank 325 may have other volumes in otherembodiments.

The plurality of isolation valves 303-310, 331, and 333 may be coupleddownstream of inlet 302. Isolation valves 303 and 305 may be part of abypass flow path 323 coupled between inlet 302 and outlet 322 thatbypasses the plurality of differently-sized flow restrictors 316-320.Isolation valve 304 may be a main verification system valve. Isolationvalves 303-310, 331, and 333 may be any suitableelectronically-controllable isolation valve capable of stopping gas flowthere through across a range of pressures created by vacuum pump 332 andthe choked flow conditions within mass flow verification system 300.

The plurality of differently-sized flow restrictors 316-320 are coupledin parallel and downstream of inlet 302. Each of the differently-sizedflow restrictors 316-320 is configured to allow a different maximum flowrate there through than the other differently-sized flow restrictors316-320. In some embodiments, for example, flow restrictor 316 may havethe highest flow rate there through, while flow restrictor 317 may havea high flow rate there through, but less than flow restrictor 316. Flowrestrictor 318 may have a medium flow rate there through (i.e., lessthan flow restrictors 316 and 317), while flow restrictor 319 may have alow flow rate there through (i.e., less than each of flow restrictors316-318). And flow restrictor 320 may have the lowest flow rate therethrough (i.e., less than each of flow restrictors 316-319). In someembodiments, the differently-sized flow restrictors 316-320 may beprecision flow restrictors. In other embodiments, standard flowrestrictors may be used.

As shown in FIG. 3, each one of the differently-sized flow restrictors316-320 is coupled in series with inlet 302 and a respective one ofisolation valves 306-310. That is, flow restrictor 316 is coupled inseries with isolation valve 306, flow restrictor 317 is coupled inseries with isolation valve 307, flow restrictor 318 is coupled inseries with isolation valve 308, flow restrictor 319 is coupled inseries with isolation valve 309, and flow restrictor 320 is coupled inseries with isolation valve 310. In some embodiments as shown, thedifferently-sized flow restrictors 316-320 are coupled downstream oftheir respective isolation valves 306-310.

In other embodiments, the number of differently-sized flow restrictorsand their respective series-coupled isolation valve may be more or lessthan that shown depending on the range of mass flow rates to be verifiedby mass flow verification system 300. The greater the range of mass flowrates to be verified, the larger the number of series-connecteddifferently-sized flow restrictor/isolation valve pairs.

Temperature sensor 311 and pressure sensor 312 may each be coupleddownstream of inlet 302 and upstream of flow restrictors 316-320.Pressure sensor 314 may be coupled downstream of flow restrictors316-320 and in particular to dead-end tank 325. Temperature sensor 311may be a thermocouple, and pressure sensors 312 and 314 may each be amanometer. In some embodiments, temperature sensor 311 may include morethan one thermocouple, and pressure sensor 312 and/or 314 may includemore than one manometer. In some embodiments, pressure sensor 312 may bea 1000 Torr manometer, and pressure sensor 314 may a 10 Torr manometer.Other embodiments may have pressure sensors of other Torr values and/ormay have more than two pressure sensors, depending on the range of massflow rates to be verified by mass flow verification system 300 and thebase vacuum pressure established by vacuum pump 332.

The gas temperature acclamation accelerator 321 may be coupled upstreamof temperature sensor 311. The gas temperature acclamation accelerator321 may be used to ensure uniform gas temperature distribution upstreamof the flow restrictors 316-320, which may improve the accuracy of massflow verification system 300. The gas temperature acclamationaccelerator 321 may be an inactive structure that includes a porous meshmaterial having an optimum amount of surface area that may result innegligible, if any, pressure drop there through.

As shown in FIG. 3, dead-end tank 325 is coupled to outlet 322 anddownstream of and in series with each one of the differently-sized flowrestrictors 316-320. Vacuum pump 332 is coupled downstream of dead-endtank 325 and in series between dead-end tank 325 and input port 334. Thedead-end tank 325, vacuum pump 332, and isolation valves 331 and 333have been included in mass flow verification system 300 to create chokedflow conditions across flow restrictors 316-320 for a sufficient periodof time during which mass flow verification may be performed, asdescribed further below.

Mass flow verification system 300 may further include a controller 324.Controller 324 may control the operation of and be electronically (orotherwise) coupled to isolation valves 303-310, 331, and 333,temperature sensor 311, pressure sensors 312 and 314, and vacuum pump332. Controller 324 may be, e.g., a general purpose computer and/or mayinclude a microprocessor or other suitable computer processor or CPU(central processing unit) capable of executing computer readableinstructions/software routines. Controller 324 may include a memory forstoring data and computer readable instructions/software routinesexecutable thereon. Flow restrictor characterization data may be storedin the memory of controller 324.

Controller 324 may be configured via user input commands and the storedcomputer readable instructions/software routines to set a set point forMFC 99C, select a flow path through one of the differently-sized flowrestrictors 316-320, control the opening and closing of each of theisolation valves 303-310, 331, and 333, set a base vacuum pressure viavacuum pump 332, record and process temperature and pressuremeasurements via temperature sensor 311 and pressure sensors 312 and314, and determine mass flow rates based on the recorded temperature andpressure measurements and Equation 2, as described herein. Controller324 may also be configured to control other aspects of mass flowverification system 300, including, e.g., input/output peripherals,power supplies, clock circuits, and/or the like.

In some embodiments, controller 324 may not be included in mass flowverification system 300. Instead, controller 324 may be, e.g., a systemcontroller of an electronic device manufacturing system to which massflow verification system 300 is connected. Data and computer readableinstructions/software routines configured to operate mass flowverification system 300 for verifying mass flow rates as describedherein may be stored on a non-transitory computer-readable medium, suchas, e.g., a removable storage disk or device. The data and computerreadable instructions/software routines may be transferred from thenon-transitory computer-readable medium to the system controller toperform mass flow verification.

Prior to mass flow verification, MFC 99C may be set to a zero set point(that is, no flow there through), isolation valves 304, any one of306-310, 331, and 333 may be opened, and vacuum pump 332 may be operatedto create a base vacuum pressure in dead-end tank 325 and at outlet 322sufficient to create choked flow conditions across differently-sizedflow restrictors 316-320 during mass flow verification. The base vacuumpressure may range in some embodiments from 200 Torr to 1 Torr, and onceachieved, isolation valves 331 and 333 may be closed.

In response to establishment of a base vacuum pressure, mass flowverification system 300 may be operated by setting MFC 99C viacontroller 324 to a desired mass flow rate (i.e., a desired set point)to be verified, selecting via controller 324 an appropriate flow paththrough one of differently-sized flow restrictors 316-320 based onstored characterization data (wherein controller 324 opens and closesthe appropriate isolation valves 303-310), measuring downstream pressurevia pressure sensor 314 (to measure the base vacuum pressure) andmeasuring upstream temperature and pressure via temperature sensor 311and pressure sensor 312. Note that the base vacuum pressure initiallyestablished in dead-end tank 325 and at outlet 322 may be temporary asgas flows through the selected flow path into dead-end tank 325. Thus,the choked flow condition across the flow restrictor in the selectedflow path may also be temporary and, accordingly, the measurements oftemperature and pressure should be made during the time that choked flowis maintained.

FIG. 3A illustrates a graph 300A of pressure versus time in mass flowverification system 300 in accordance with one or more embodiments ofthe disclosure. Pressure curve 381 represents a pressure measured bydownstream pressure sensor 314 at dead-end tank 325, and pressure curve382 represents a pressure measured by upstream pressure sensor 312.Shortly after gas begins flowing through MFC 99C (i.e., at about 1 sec),the vacuum pressure in dead-end tank 325 begins to steadily rise, asshown by pressure curve 381. Pressure measured by upstream pressuresensor 312 also rises, but then remains constant for about 4.5 seconds,in some embodiments, before rising again, as shown by pressure curve382. Choked flow occurs during this constant pressure choked flowduration 383. Accordingly, upstream temperature and pressuremeasurements should be made during this time which, in some embodiments,is between about 2.0 to 6.5 seconds after initiating gas flow throughMFC 99C.

Furthermore, because pressure sensor 312 is coupled upstream ofisolation valves 306-310, which are upstream of differently-sized flowrestrictors 316-320, pressure measurements made by pressure sensor 312may not be the same as those that would have been made directly upstreamof differently-sized flow restrictors 316-320 (i.e., between a flowrestrictor and its respective isolation valve). FIG. 3A also includes apressure curve 384 that represents a pressure that would be measureddirectly upstream of one of flow restrictors 316-320 if a respectivepressure sensor were located between each one of the differently-sizedflow restrictors 316-320 and isolation valves 306-310. Although thedifference between the two pressures may be small, as indicated bypressure curves 382 and 384 (e.g., about 10 Torr in some embodiments),the difference may be enough to adversely affect the accuracy of thedetermined mass flow rate for verification purposes.

Therefore, in order to determine an accurate mass flow rate, thepressure measured by pressure sensor 312 may be converted to a pressurevalue that would have been measured had a pressure sensor been locateddirectly upstream of the flow restrictor in the selected flow path. Insome embodiments, a suitable model-based computation algorithm based onknown fluid mechanics may be used to account for the pressure differencebetween the location of pressure sensor 312 and locations directlyupstream of each one of differently-sized flow restrictors 316-320.

The data shown in FIG. 3A may be based on analyses of an MFC set pointof 100 slm of hydrogen ramping up in one second, a 25 liter dead-endtank with a 2.5 mm orifice and a 0.7 conductance valve, 100 Torr initialbase vacuum pressure, and pressure downstream of the MFC maintainedunder 600 Torr to prevent MFC starvation.

In response to measuring the upstream temperature and converting themeasured upstream pressure, controller 324 may determine a mass flowrate using Equation 2. This process may be repeated for verifying othermass flow rates of MFC 99C. If a determined mass flow rate is found tobe outside of the specified accuracy of MFC 99C, MFC 99C may be adjusted(if possible) or replaced.

In some embodiments, mass flow verification system 300 may not includedead-end tank 325, wherein vacuum pump 332 may be directly coupled tooutlet 322. In these embodiments, vacuum pump 332 may be capable of 50slm or higher continuous flow, which may be sufficient to maintain astable base vacuum pressure during verification.

FIG. 4 illustrates another mass flow verification system 400 inaccordance with one or more embodiments. Mass flow verification system400 may be used in atmospheric applications (i.e., non-vacuumapplications) and, alternatively, may also be used in reduced-pressureapplications.

A mass flow controller (MFC) 99D may be coupled to mass flowverification system 400 at inlet 402 of mass flow verification system400. In some embodiments, MFC 99D may represent a plurality of MFCscoupled to inlet 402 via a common manifold or header having a commonoutlet, wherein MFC 99D as described below may represent the one MFC ofthe plurality of MFCs to be verified (i.e., the only MFC of theplurality of MFCs flowing gas during verification). MFC 99D may be apart of, or coupled to, a gas delivery apparatus of an electronic devicemanufacturing system. MFC 99D may be configured to flow a gas at one ormore specified mass flow rates (i.e., one or more set points) to one ormore process chambers of the electronic device manufacturing system.Mass flow verification system 400 is configured to verify one or more ofthe specified mass flow rates of MFC 99D based on choked flowprinciples.

Mass flow verification system 400 may include a plurality of isolationvalves 403-410, 426-430, 431, and 433; a temperature sensor 411;pressure sensors 412 and 414, a plurality of differently-sized flowrestrictors 416-420, a gas temperature acclamation accelerator 421, anoutlet 422, a dead-end tank 425, a vacuum pump 432, and an input port434.

Input port 434 may be coupled to a source of CDA (clean dry air) ornitrogen. Vacuum pump 432 may be, e.g., a compact vacuum pump, and mayhave an exhaust port 435 that may be coupled to an abatement system ofan electronic device manufacturing system or other suitable apparatusfor receiving discharged gases. Dead-end tank 425 may have a volume of,e.g., about 25 liters. Dead-end tank 425 may have other volumes in otherembodiments.

The plurality of isolation valves 403-410, 426-430, 431, and 433 may becoupled downstream of inlet 402. Isolation valves 403 and 405 may bepart of a bypass flow path 423 coupled between inlet 402 and outlet 422that bypasses the plurality of differently-sized flow restrictors416-420. Isolation valve 404 may be a main verification system valve.Isolation valves 403-410, 426-430, 431, and 433 may be any suitableelectronically-controllable isolation valve capable of stopping gas flowthere through across a range of pressures created by vacuum pump 432 andthe choked flow conditions within mass flow verification system 400.

The plurality of differently-sized flow restrictors 416-420 are coupledin parallel and downstream of inlet 402. Each of the differently-sizedflow restrictors 416-420 is configured to allow a different maximum flowrate there through than the other differently-sized flow restrictors416-420. In some embodiments, for example, flow restrictor 416 may havethe highest flow rate there through, while flow restrictor 417 may havea high flow rate there through, but less than flow restrictor 416. Flowrestrictor 418 may have a medium flow rate there through (i.e., lessthan flow restrictors 416 and 417), while flow restrictor 419 may have alow flow rate there through (i.e., less than each of flow restrictors416-418). And flow restrictor 420 may have the lowest flow rate therethrough (i.e., less than each of flow restrictors 416-419). In someembodiments, the differently-sized flow restrictors 416-420 may beprecision flow restrictors. In other embodiments, standard flowrestrictors may be used.

As shown in FIG. 4, each one of the differently-sized flow restrictors416-420 is coupled in series with inlet 402 and a respective one ofisolation valves 406-410. That is, flow restrictor 416 is coupled inseries with isolation valve 406, flow restrictor 417 is coupled inseries with isolation valve 407, flow restrictor 418 is coupled inseries with isolation valve 408, flow restrictor 419 is coupled inseries with isolation valve 409, and flow restrictor 420 is coupled inseries with isolation valve 410. In some embodiments as shown, thedifferently-sized flow restrictors 416-420 are coupled downstream oftheir respective isolation valves 406-410.

In other embodiments, the number of differently-sized flow restrictorsand their respective series-coupled isolation valve may be more or lessthan that shown depending on the range of mass flow rates to be verifiedby mass flow verification system 400. The greater the range of mass flowrates to be verified, the larger the number of series-connecteddifferently-sized flow restrictor/isolation valve pairs.

Temperature sensor 411 and pressure sensor 412 may each be coupleddownstream of inlet 402 and upstream of the differently-sized flowrestrictors 416-420. Pressure sensor 414 may be coupled downstream ofthe differently-sized flow restrictors 416-420 and in particular todead-end tank 425. As shown in FIG. 4, each one of the sub-plurality ofisolation valves 426-430 has a respective first port 436-439 (exceptisolation valve 430 which shares first port 439 with isolation valve429) coupled to temperature sensor 411 and to pressure sensor 412. Eachone of the sub-plurality of isolation valves 426-430 also has a secondport coupled between a respective one of the differently-sized flowrestrictors 416-420 and a respective one of isolation valves 406-410.That is, isolation valve 426 has a second port 446 coupled between flowrestrictor 416 and isolation valve 406, isolation valve 427 has a secondport 447 coupled between flow restrictor 417 and isolation valve 407,isolation valve 428 has a second port 448 coupled between flowrestrictor 418 and isolation valve 408, isolation valve 429 has a secondport 449 coupled between flow restrictor 419 and isolation valve 409,and isolation valve 430 has a second port 450 coupled between flowrestrictor 420 and isolation valve 410.

In some embodiments, the sub-plurality of isolation valves 426-430 mayeach be a suitable mini-valve configured to allow temperature sensor 411and pressure sensor 412 to accurately measure temperature and pressure,respectively, directly upstream of each one of the differently-sizedflow restrictors 416-420 when its corresponding isolation valve 426-430is open.

Temperature sensor 411 may be a thermocouple, and pressure sensors 412and 414 may each be a manometer. In some embodiments, temperature sensor411 may include more than one thermocouple, and pressure sensor 412and/or 414 may include more than one manometer. In some embodiments,pressure sensor 412 may be a 1000 Torr manometer, and pressure sensor414 may a 10 Torr manometer. Other embodiments may have pressure sensorsof other Torr values and/or may have more than two pressure sensors,depending on the range of mass flow rates to be verified by mass flowverification system 400.

The gas temperature acclamation accelerator 421 may be coupled upstreamof temperature sensor 411. The gas temperature acclamation accelerator421 may be used to ensure uniform gas temperature distribution upstreamof the flow restrictors 416-420, which may improve the accuracy of massflow verification system 400. The gas temperature acclamationaccelerator 421 may be an inactive structure that includes a porous meshmaterial having an optimum amount of surface area that may result innegligible, if any, pressure drop there through.

As shown in FIG. 4, dead-end tank 425 is coupled to outlet 422 anddownstream of and in series with each one of the differently-sized flowrestrictors 416-420. Vacuum pump 432 is coupled downstream of dead-endtank 425 and in series between dead-end tank 425 and input port 434. Thedead-end tank 425, vacuum pump 432, and isolation valves 431 and 433have been included in mass flow verification system 400 to create chokedflow conditions across flow restrictors 416-420 for a sufficient periodof time during which mass flow verification may be performed, asdescribed further below.

Mass flow verification system 400 may further include a controller 424.Controller 424 may control the operation of and be electronically (orotherwise) coupled to isolation valves 403-410, 426-430, 431, and 433;temperature sensor 411; pressure sensors 412 and 414, and vacuum pump432. Controller 424 may be, e.g., a general purpose computer and/or mayinclude a microprocessor or other suitable computer processor or CPU(central processing unit) capable of executing computer readableinstructions/software routines. Controller 424 may include a memory forstoring data and computer readable instructions/software routinesexecutable thereon. Flow restrictor characterization data may be storedin the memory of controller 424.

Controller 424 may be configured via user input commands and the storedcomputer readable instructions/software routines to set a set point forMFC 99D, select a flow path through one of the differently-sized flowrestrictors 416-420, control the opening and closing of each of theisolation valves 403-410, 426-430, 431, and 433; set a base vacuumpressure via vacuum pump 432; record and process temperature andpressure measurements via temperature sensor 411 and pressure sensors412 and 414; and determine of mass flow rates based on the recordedtemperature and pressure measurements and Equation 2, as describedherein. Controller 424 may also be configured to control other aspectsof mass flow verification system 400, including, e.g., input/outputperipherals, power supplies, clock circuits, and/or the like.

In some embodiments, controller 424 may not be included in mass flowverification system 400. Instead, controller 424 may be, e.g., a systemcontroller of an electronic device manufacturing system to which massflow verification system 400 is connected. Data and computer readableinstructions/software routines configured to operate mass flowverification system 400 for verifying mass flow rates as describedherein may be stored on a non-transitory computer-readable medium, suchas, e.g., a removable storage disk or device. The data and computerreadable instructions/software routines may be transferred from thenon-transitory computer-readable medium to the system controller toperform mass flow verification.

Prior to mass flow verification, MFC 99D may be set to a zero set point(that is, no flow there through), isolation valves 404, any one of406-410, 431, and 433 may be opened, and vacuum pump 432 may be operatedto create a base vacuum pressure in dead-end tank 425 and at outlet 422sufficient to create choked flow conditions across the differently-sizedflow restrictors 416-420 during mass flow verification. The base vacuumpressure may range in some embodiments from 200 Torr to 1 Torr, and onceachieved, isolation valves 431 and 433 may be closed.

In response to establishment of a base vacuum pressure, mass flowverification system 400 may be operated by setting MFC 99D viacontroller 424 to a desired mass flow rate (i.e., a desired set point)to be verified, selecting via controller 424 an appropriate flow paththrough one of differently-sized flow restrictors 416-420 based onstored characterization data (wherein controller 424 opens and closesthe appropriate isolation valves 403-410 and 426-430), measuringdownstream pressure via pressure sensor 414 (to measure the base vacuumpressure), and measuring upstream temperature and pressure viatemperature sensor 411 and pressure sensor 412.

As in mass flow verification system 300, these measurements should bemade during the time that choked flow is maintained via dead-end tank425, as illustrated by choked flow duration 383 of FIG. 3A, which insome embodiments may be about 2.0-6.5 seconds after initiating gas flowthrough MFC 99D. This choked flow duration may be based on analyses ofan MFC set point of 100 slm of hydrogen ramping up in one second, a 25liter dead-end tank with a 2.5 mm orifice and a 0.7 conductance valve,100 Torr initial base vacuum pressure, and pressure downstream of theMFC maintained under 600 Torr to prevent MFC starvation.

Because pressure sensor 412 is coupled such that pressure measurementscan be made directly upstream of the differently-sized flow restrictors416-420, the pressure measurements do not need to be converted viamodel-based computation algorithms as in mass flow verification system300.

In response to measuring upstream temperature and pressure, controller424 may determine a mass flow rate using Equation 2. This process may berepeated for verifying other mass flow rates of MFC 99D. If a determinedmass flow rate is found to be outside of the specified accuracy of MFC99D, MFC 99D may be adjusted (if possible) or replaced.

In some embodiments, mass flow verification system 400 may not includedead-end tank 325, wherein vacuum pump 432 may be directly coupled tooutlet 422. In these embodiments, vacuum pump 432 may be capable of 50slm or higher continuous flow, which may be sufficient to maintain astable base vacuum pressure during verification.

Although mass flow verification systems 300 and 400 include their ownvacuum pump for atmospheric applications, each may be used in anelectronic device manufacturing system employing its own system vacuumpump for reduced-pressure applications. In some reduced-pressureapplications, connection from a system vacuum pump of an electronicdevice manufacturing system to a mass flow verification system outlet,such as, e.g., outlets 122 or 222, may not be direct. Instead, such aconnection may involve several flow path restrictions and/or othercomplexities within the electronic device manufacturing system that mayadversely affect the ability of the system vacuum pump to provide andmaintain a stable and satisfactory base vacuum pressure to enable massflow verification during choked flow in either mass flow verificationsystem 100 or 200. Thus, the dead-end tank and vacuum pump arrangementof mass flow verification systems 300 and/or 400 may allow those systemsto be alternatively used in such reduced-pressure applications toinitially and quickly achieve a stable base vacuum pressure thattemporarily maintains choked flow for mass flow verification.

FIG. 5 illustrates an electronic device manufacturing system 500 inaccordance with one or more embodiments. Electronic device manufacturingsystem 500 may include an MFC 599, a mass flow verification system 560,and a process chamber 570. In some embodiments, MFC 599 may represent aplurality of MFCs coupled via a common manifold or header to a commonoutlet, wherein MFC 599 as described below may represent the one MFC ofthe plurality of MFCs to be verified (i.e., the only MFC of theplurality of MFCs flowing gas during verification).

Process chamber 570 may be coupled to a flow path 572 coupled to massflow controller 599 via an isolation valve 573. Process chamber 570 maybe configured to receive one or more process chemistries via MFC 599 andto have a reduced-pressure chemical vapor deposition process, or areduced-pressure epitaxy process, or one or more deposition, oxidation,nitration, etching, polishing, cleaning, and/or lithography processesperformed therein.

Mass flow verification system 560 may have an inlet 502 and an outlet522. Inlet 502 may be coupled to MFC 599 via isolation valve 573. Massflow verification system 560 may be any one of mass flow verificationsystems 100, 200, 300, or 400.

In those embodiments where electronic device manufacturing system 500operates under a reduced-pressure application, mass flow verificationsystem 560 may be any one of mass flow verification systems 100, 200,300, or 400. Mass flow verification system 560 may be coupled via outlet522 to a system vacuum pump 565 of electronic device manufacturingsystem 500 via an isolation valve 575. System vacuum pump 565 may alsobe coupled to process chamber 570 via isolation valve 575.

In those embodiments where electronic device manufacturing system 500operates under an atmospheric application, mass flow verification system560 may be mass flow verification system 300 or 400. In theseembodiments, system vacuum pump 565 may be excluded from electronicdevice manufacturing system 500.

The operation of electronic device manufacturing system 500 and/or massflow verification system 560 may be controlled by a controller such as,e.g., one of controllers 124, 224, 324, or 424.

FIG. 6 illustrates a method 600 of verifying a mass flow rate inaccordance with one or more embodiments. At process block 602, method600 may include causing a gas to flow through only one of a plurality ofdifferently-sized flow restrictors during a choked flow condition. Forexample, referring to FIG. 2, controller 224 may set MFC 99B to aparticular mass flow rate, close isolation valves 205 and 207-210, openisolation valve 203 for flow through main flow path 201, and openisolation valves 204 and 206 to allow gas to flow from MFC 99B throughonly flow restrictor 216.

At process block 604, a pressure upstream of the one of the plurality ofdifferently-sized flow restrictors may be measured to obtain a measuredpressure value during the choked flow condition. For example, againreferring to FIG. 2, controller 224 may open isolation valve 226 andclose isolation valves 227-230 and receive a measured pressure valuefrom pressure sensor 212.

At process block 606, method 600 may include measuring a temperatureupstream of the one of the plurality of differently-sized flowrestrictors to obtain a measured temperature value during the chokedflow condition. Continuing with the FIG. 2 example, controller 224 mayreceive a measured temperature value from temperature sensor 211.

And at process block 606, method 600 may include determining a mass flowrate by applying a predetermined flow restrictor coefficient, apredetermined gas correction factor, and a predetermined temperaturevalue to the measured pressure value and the measured temperature value.For example, controller 224 may determine a mass flow rate based on themeasured values of pressure and temperature by applying Equation 2 andthe appropriate predetermined flow restrictor coefficient, predeterminedgas correction factor, and predetermined temperature value stored in amemory of controller 224 for flow restrictor 216 and the particular gasflowed there through.

The above process blocks of method 600 may be executed or performed inan order or sequence not limited to the order and sequence shown anddescribed. For example, in some embodiments, process block 604 may beperformed simultaneously with or after process block 606 (in such cases,the appropriate isolation values 226-230 are opened and closed asdescribed for process block 604 prior to any temperature and/or pressuremeasurement).

In some embodiments, a non-transitory computer-readable medium, such as,e.g., a removable storage disk or device, may include computer readableinstructions stored thereon that are capable of being executed byprocessor, such as, e.g., controllers 124-424, to perform process blocks602, 604, 606, and 608 of method 600.

The foregoing description discloses only example embodiments of thedisclosure. Modifications of the above-disclosed assemblies, apparatus,systems, and methods may fall within the scope of the disclosure.Accordingly, while example embodiments of the disclosure have beendisclosed, it should be understood that other embodiments may fallwithin the scope of the disclosure, as defined by the following claims.

What is claimed is:
 1. A mass flow verification system, comprising: aninlet to input a flow of a target gas; a first pressure sensor tomeasure, downstream of the inlet, a first pressure value of the targetgas; a first flow restrictor coupled downstream of the inlet; a secondflow restrictor coupled downstream of the inlet and in parallel to thefirst flow restrictor, wherein the second flow restrictor is sizeddifferently than the first flow restrictor; a first isolation valvecoupled downstream of the inlet and in series with the first flowrestrictor; a second isolation valve coupled downstream of the inlet andin series with the second flow restrictor; a third isolation valvehaving a first port and a second port, the first port of the thirdisolation valve coupled to the first pressure sensor and the second portof the third isolation valve coupled between the first isolation valveand the first flow restrictor; an outlet coupled downstream of and inseries with the first flow restrictor and the second flow restrictor;and a controller coupled to the first pressure sensor, the firstisolation valve, the second isolation valve, and the third isolationvalve, wherein the controller is configured to: control the firstisolation valve to cause the first isolation valve to be open and tocause a choked flow of the target gas through the first flow restrictorto be established; control the second isolation valve to cause thesecond isolation valve to be closed; control the third isolation valveto cause the third isolation valve to be open, wherein no flow of thetarget gas is to occur through the third isolation valve after thechoked flow through the first flow restrictor is established; obtain,from the first pressure sensor, the first pressure value of the targetgas between the first isolation valve and the first flow restrictor;determine, using the first pressure value of the target gas, a mass flowrate of the target gas.
 2. The mass flow verification system of claim 1,further comprising a second pressure sensor to measure, downstream ofthe first flow restrictor, a second pressure value of the target gas,and wherein the controller is coupled to the second pressure sensor andis further configured to: confirm, based on a ratio of the firstpressure value to the second pressure value, that the flow of the targetgas through the first flow restrictor is occurring under the choked flowcondition.
 3. The mass flow verification system of claim 1, wherein tocause the first isolation valve to be open and the second isolationvalve to be closed the controller is configured to: receive instructionsto verify a set mass flow rate of the flow of the target gas input onthe mass flow verification system; and determine that the firstisolation valve is to be open responsive to the set mass flow rate andcharacterization data indicating that: the set mass flow rate of thetarget gas through the first flow restrictor is to occur under thechoked flow condition; and the first pressure value is not to exceed athreshold value; and determine that the second isolation valve is to beclosed responsive to determining that the first isolation valve is to beopen.
 4. The mass flow verification system of claim 3, wherein thethreshold value is 600 Torr.
 5. The mass flow verification system ofclaim 1, further comprising a temperature sensor coupled to the firstport of the third isolation valve, and wherein the controller is furtherconfigured to obtain, from the temperature sensor, a temperature valueof the target gas between the first isolation valve and the first flowrestrictor, and wherein to determine the mass flow rate of the targetgas, the controller is further to use the temperature value of thetarget gas.
 6. The mass flow verification system of claim 1, wherein todetermine the mass flow of the target gas the controller is furtherconfigured to correct for a difference between a molecular weight of thetarget gas and a molecular weight of a reference gas.
 7. The mass flowverification system of claim 1, wherein the reference gas is nitrogen.8. The mass flow verification system of claim 1, further comprising agas temperature acclamation accelerator coupled upstream of the firstflow restrictor.
 9. The mass flow verification system of claim 1,further comprising: a tank coupled downstream of the first flowrestrictor; and a vacuum pump coupled to the tank.
 10. The mass flowverification system of claim 1, further comprising a vacuum pump coupledto the outlet, the vacuum pump having an exhaust port.
 11. The mass flowverification system of claim 1, wherein the first pressure sensorcomprises one or more manometers.
 12. The mass flow verification systemof claim 1, further comprising a fourth isolation valve having a firstport coupled to the first pressure sensor and a second port coupledbetween the second isolation valve and the second flow restrictor. 13.The mass flow verification system of claim 9, wherein the controller isfurther configured to: prevent the target gas from flowing through theinlet; control the vacuum pump to create a base vacuum pressure in thetank; allow the target gas to flow through the inlet; monitor, by thefirst pressure sensor, the first pressure value of the target gas;identify a limited-duration period of a choked flow of the target gasthrough the first flow restrictor; and wherein the first pressure valueof the target gas is a pressure value determined during thelimited-duration period of the choked flow.
 14. A mass flow verificationsystem of claim 13, wherein to identify the limited-duration period ofthe choked flow of the target gas through the first flow restrictor, thecontroller is further to monitor a pressure value of the target gasinside the tank using a second pressure sensor coupled to the tank.